i965: Do dead-code elimination in a single pass.

i965: Do dead-code elimination in a single pass.

The first pass marked dead instructions as opcode = NOP, and a second
pass deleted those instructions so that the live ranges used in the
first pass wouldn't change.

But since we're walking the instructions in reverse order, we can just
do everything in one pass. The only thing we have to do is walk the
blocks in reverse as well.

Reviewed-by: Francisco Jerez <currojerez@riseup.net>